Conventional rigid cannulae which are used in endoscopic operations consist of a tube, whose distal zone is introducible into the inside of the body through a trocar puncture point, and a top piece which is attached to the proximal end of the tube. At its proximal end face the top piece has an opening, essentially aligned with the cross-section surface of the tube, to which is allocated a seal engaging at the shaft of introduced operation instruments. Located in the inside of the top piece is a closure device for the separate closing of the opening. Additional equipment, for example: (1) a trocar with which the puncture point is produced in the body wall and which is withdrawn from the cannula following the insertion of the cannula into the body wall; or (2) a reduction cap with seal for the use of instruments whose external diameter is much smaller than the internal diameter of the cannula, can be secured on the proximal end face of the top piece with the help of coupling elements.
In the case of operations in the abdominal area in particular, a gas is introduced into the inside of the body in order that the organs detach from the body wall. A gas connection with a valve can be provided at the top piece of the cannula for this purpose. If no surgical instrument is introduced into the cannula, the closure device is closed, so that the compressed gas cannot escape. Upon insertion of an instrument, the aforementioned seal engages at its shaft so that an escape of gas is similarly prevented.
A disadvantage with the previously known cannula is that it does not permit the introduction of curved instruments. At best, very thin and only slightly curved instruments can be inserted into a rigid large-diameter cannula. Attempts are thus being made to develop instruments having joints or hinges which are straight when guided through the cannula and whose distal zone is then bent in the inside of the body. Instruments with hinges are of a very costly design, however; for example, they require internal actuating systems in order to allow movement in the distal zone. Such instruments consequently have a larger shaft diameter, an can thus be introduced only through large cannulae which are generally less advantageous for the patient than small-diameter cannulae. Their elaborate design also makes them more expensive to produce.
Cannulae with a flexible tube are known for operations in the thorax region. Upon entry of air into the inside of the thorax, the wings of the lung change their shape and the organs are readily accessible for the operation; hence no compressed gas is required for such operations. There is thus no need for a separate closure device in the top piece of the cannula, so that the top piece can be designed without difficulties. A rigid design of the top piece is also possible in this case, which, together with a flat design, does not impede the introduction of curved operation instruments. By contrast, the top piece of the initially described conventional rigid cannula has a substantial structural height, so that simply replacing the conventional rigid tube by a flexible tube would not result in a flexible cannula which permits the insertion of curved instruments.